refactor(backend): Reduce circular imports (#12068)

I'm getting circular import issues because there is a lot of
cross-importing between `backend.data`, `backend.blocks`, and other
modules. This change reduces block-related cross-imports and thus risk
of breaking circular imports.

### Changes 🏗️

- Strip down `backend.data.block`
- Move `Block` base class and related class/enum defs to
`backend.blocks._base`
  - Move `is_block_auth_configured` to `backend.blocks._utils`
- Move `get_blocks()`, `get_io_block_ids()` etc. to `backend.blocks`
(`__init__.py`)
  - Update imports everywhere
- Remove unused and poorly typed `Block.create()`
  - Change usages from `block_cls.create()` to `block_cls()`
- Improve typing of `load_all_blocks` and `get_blocks`
- Move cross-import of `backend.api.features.library.model` from
`backend/data/__init__.py` to `backend/data/integrations.py`
- Remove deprecated attribute `NodeModel.webhook`
  - Re-generate OpenAPI spec and fix frontend usage
- Eliminate module-level `backend.blocks` import from `blocks/agent.py`
- Eliminate module-level `backend.data.execution` and
`backend.executor.manager` imports from `blocks/helpers/review.py`
- Replace `BlockInput` with `GraphInput` for graph inputs

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
  - CI static type-checking + tests should be sufficient for this

docs/platform/new_blocks.md

@@ -20,13 +20,13 @@ Follow these steps to create and test a new block:
    Every block should contain the following:
 
    ```python
-   from backend.data.block import Block, BlockSchemaInput, BlockSchemaOutput, BlockOutput
+   from backend.blocks._base import Block, BlockSchemaInput, BlockSchemaOutput, BlockOutput
    ```
 
    Example for the Wikipedia summary block:
 
    ```python
-   from backend.data.block import Block, BlockSchemaInput, BlockSchemaOutput, BlockOutput
+   from backend.blocks._base import Block, BlockSchemaInput, BlockSchemaOutput, BlockOutput
    from backend.utils.get_request import GetRequest
    import requests
 
@@ -237,7 +237,7 @@ from backend.data.model import (
     Credentials,
 )
 
-from backend.data.block import Block, BlockOutput, BlockSchemaInput, BlockSchemaOutput
+from backend.blocks._base import Block, BlockOutput, BlockSchemaInput, BlockSchemaOutput
 from backend.data.model import CredentialsField
 from backend.integrations.providers import ProviderName
 
@@ -496,8 +496,8 @@ To create a webhook-triggered block, follow these additional steps on top of the
    <details>
    <summary><code>BlockWebhookConfig</code> definition</summary>
 
-   ```python title="backend/data/block.py"
-   --8<-- "autogpt_platform/backend/backend/data/block.py:BlockWebhookConfig"
+   ```python title="backend/blocks/_base.py"
+   --8<-- "autogpt_platform/backend/backend/blocks/_base.py:BlockWebhookConfig"
    ```
 
    </details>